An underwater sound-absorbing metamaterial

Abstract

The present invention discloses an underwater sound-absorbing metamaterial, belonging to the technical field of underwater sound absorption. The underwater sound-absorbing metamaterial includes periodically arranged unit cells. The unit cell is a sandwich structure. Along the incident propagation direction of the sound wave, the upper and lower layers are low water absorption fiber-reinforced resin panels, and the middle is a high-damping polymer embedded with a cavity and a high-density mass block. The high-damping polymer contains two cylindrical cavities, upper and lower. The high-density mass block is located between the two cavities and is not directly connected to the cavities. The diameter of the upper cavity is lower than that of the lower cavity, and both are larger than the diameter of the high-density mass block. The ratio of the height of the high-density mass block to the distance between the upper and lower cavities is 0.8 to 0.9. The unit cell structure of the underwater sound-absorbing metamaterial of the present invention is simple, the raw materials are easy to obtain, and it is easy to prepare. The unit cell parameters are highly designable, and the sound absorption performance of the metamaterial can be flexibly adjusted to meet the integrated requirements of lightweight and low-frequency broadband sound absorption.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater sound absorption, and particularly relates to an underwater sound-absorbing metamaterial. Background Art

[0002] To improve underwater stealth and survivability, underwater equipment usually needs to be covered with an acoustic coating on its surface to isolate its own noise from radiating outward, and at the same time reduce its acoustic target strength by absorbing the sound waves of an active sonar. The sound-absorbing coating polymer containing cavities is one of the main structures for low-frequency sound absorption. Taking the Alberich-type structure as a typical example, there are several layers of periodically arranged cylindrical cavities distributed inside the polymer material, and the cavity sizes and distributions of each layer are different. The polymer uses rubber or polyurethane with a characteristic impedance similar to that of water to achieve impedance matching, so that sound waves can enter the sound-absorbing material interior to the greatest extent, thereby achieving effective sound absorption in a wide frequency band. However, since the sound-absorbing frequency of the resonant cavity is approximately inversely proportional to the cube root of the cavity volume, low-frequency sound absorption often requires large-scale cavities to achieve, and the cylindrical cavity also has the disadvantage of a narrow resonant sound-absorbing frequency. The cavity with an "inverted horn" shape has the advantages of both impedance gradient and resonant sound absorption, which can further expand the broadband sound-absorbing effect. Introducing local resonance units inside the polymer can effectively achieve low-frequency and high-efficiency sound absorption, and the operating frequency can be reduced to 1 kHz or even lower, which can counter the detection threat of low-frequency active sonars.

[0003] However, currently, the sound-absorbing coating usually directly exposes the rubber polymer material to the marine environment. On the one hand, it needs to directly bear high hydrostatic pressure, resulting in the deformation of the cavities inside the polymer and a decrease in sound-absorbing performance; and when the cavity size inside the polymer is large, it is also easy to cause deformation concentration, and repeated deformation will lead to irregular changes in the shape of the sound-absorbing coating, increasing the navigation resistance. On the other hand, the polymer exposed to seawater is more likely to be eroded by seawater over the years, which can cause corrosion damage, and in severe cases, it will even cause the sound-absorbing coating to fall off, making it lose its acoustic protection function. Summary of the Invention

[0004] The purpose of the present invention is to provide an underwater sound-absorbing metamaterial, which uses a low water absorption fiber-reinforced resin panel to isolate the polymer from seawater, solves the technical problems that the traditional polymer containing cavities is directly exposed to the marine environment and has large local deformation of the polymer when bearing water pressure, resulting in uneven surface and decreased acoustic performance, and at the same time maintains good low-frequency broadband sound-absorbing performance.

[0005] To achieve the above purpose and solve the above technical problems, the technical solution of the present invention is as follows:

[0006] The present invention provides an underwater sound-absorbing metamaterial, which includes periodically arranged unit cells. The unit cell has a sandwich structure. Along the direction of sound wave incidence and propagation, the upper and lower layers are low water absorption fiber-reinforced resin panels, and the middle is a high-damping polymer embedded with a cavity and a high-density mass block. The high-damping polymer contains two cylindrical cavities, the upper and lower ones. The high-density mass block is located between the two cavities and is not directly connected to the cavities. The diameter of the upper cavity is lower than that of the lower cavity, and both are larger than the diameter of the high-density mass block. The ratio of the height of the mass block to the distance between the upper and lower cavities is 0.8 to 0.9.

[0007] Furthermore, the cavities and the high-density mass block inside the high-damping polymer are located at the center of the unit cell in the horizontal direction and are axially symmetrically distributed along the longitudinal axis of the unit cell in the direction of sound wave incidence.

[0008] Furthermore, the height of the high-damping polymer in the unit cell is 50 mm, which is made of modified rubber or modified polyurethane, with an elastic modulus of 20 to 30 MPa and an isotropic loss factor of 0.5 to 0.6.

[0009] Furthermore, the thickness of the upper and lower low water absorption fiber-reinforced resin panels is 1 to 2 mm, which is made of continuous glass fiber-reinforced unsaturated vinyl resin material.

[0010] The effective benefits of the present invention are as follows:

[0011] 1. For the underwater sound-absorbing metamaterial provided by the present invention, the low water absorption fiber-reinforced resin panel is used to isolate the direct contact between the damping polymer and seawater, enhancing the marine corrosion resistance and erosion resistance of the sound-absorbing covering layer. At the same time, the low water absorption can reduce seawater penetration, improve the service performance of the material, and increase the service time.

[0012] 2. For the underwater sound-absorbing metamaterial provided by the present invention, the combination of cavity-mass block-cavity is used to achieve low-frequency broadband sound-absorbing performance. Especially by controlling the distance between the end face of the mass block and the cross-sections of the upper and lower cavities, the low-frequency sound-absorbing performance is effectively improved.

[0013] 3. For the underwater sound-absorbing metamaterial provided by the present invention, it has good broadband sound-absorbing performance under both steel backing and water backing conditions, providing a new choice for underwater sound-absorbing materials.

[0014] 4. For the underwater sound-absorbing metamaterial provided by the present invention, the unit cell structure is simple, the raw materials are easy to obtain, and it is easy to prepare. The unit cell unit parameters have strong designability, and the sound-absorbing performance of the metamaterial can be flexibly adjusted to meet the integrated requirements of lightweight and low-frequency broadband sound absorption. Description of the Drawings

[0015] Figure 1 is a schematic diagram of a unit cell of an underwater sound-absorbing metamaterial resistant to the marine environment.

[0016] Figure 2 is the central axisymmetric sectional view of the unit cell of the underwater sound-absorbing metamaterial resistant to the marine environment

[0017] Where: 1 - upper low water absorption fiber-reinforced resin panel; 2 - sound-absorbing polymer; 3 - upper cavity; 4 - high-density mass block; 5 - lower cavity; 6 - lower low water absorption fiber-reinforced resin panel.

[0018] Figure 3 are the absorption, reflection and transmittance of the underwater sound-absorbing metamaterial with the fiber-reinforced resin panel thickness of 1 mm in Example 1 at 1 - 10 kHz.

[0019] Figure 4 are the absorption, reflection and transmittance of the underwater sound-absorbing metamaterial with the fiber-reinforced resin panel thickness of 2 mm in Example 1 at 1 - 10 kHz.

[0020] Figure 5 is the comparison curve of the absorption rate between the underwater sound-absorbing metamaterial in Example 1 and the sandwich damping polymer material.

[0021] Figure 6 are the absorption, reflection and transmittance of the underwater sound-absorbing metamaterial with the fiber-reinforced resin panel thickness of 2 mm in Example 2 at 1 - 10 kHz.

[0022] Figure 7 is the comparison curve of the absorption rate between the underwater sound-absorbing metamaterial in Example 2 and the sandwich damping polymer material. Detailed implementation manners

[0023] The present invention aims to provide an underwater sound-absorbing metamaterial, which adopts the sound-absorbing combination form of cavity - mass block - cavity, and realizes the low-frequency broadband sound-absorbing performance by adjusting the diameter and height parameters of the upper and lower cavities and the distance between the end face of the mass block and the cross section of the upper and lower cavities. At the same time, the low water absorption fiber-reinforced resin panel is adopted to enhance the corrosion resistance and erosion resistance in the marine environment, and it can be used in the environment with a steel back lining or water back linings at both ends.

[0024] For a better understanding of the present invention, the present invention will be explained and described in detail below with reference to the drawings and embodiments.

[0025] The present invention provides an underwater sound-absorbing metamaterial, which includes periodically arranged unit cells. The unit cell is a sandwich structure. Along the incident propagation direction of the sound wave, the upper and lower layers are low water absorption fiber-reinforced resin panels, and the middle is a high-damping polymer embedded with a cavity and a high-density mass block. The high-damping polymer contains two cylindrical cavities, the upper and the lower. The high-density mass block is located between the two cavities and is not directly connected to the cavities. The diameter of the upper cavity is smaller than that of the lower cavity, and both are larger than the diameter of the high-density mass block. The ratio of the height of the mass block to the distance between the upper and lower cavities is 0.8 to 0.9.

[0026] Furthermore, by adopting a cavity combination form where the upper cavity has a small diameter and a large length, and the lower cavity has a large diameter and a small thickness, an impedance gradient change inside the sound-absorbing polymer can be achieved, and the scattering and waveform conversion effects of the cavities on the sound wave can be increased. Different resonance absorption frequencies also contribute to improving the sound absorption bandwidth. The present invention also controls the distance between the end face of the mass block and the cross section of the upper and lower cavities to a small value. The presence of the cavities is equivalent to reducing the elastic modulus of the polymer at the end face, thereby enhancing the resonance effect of the mass block, increasing the displacement and energy loss of the up and down vibration at the resonance frequency, and thus improving the low-frequency sound absorption performance of the metamaterial.

[0027] Furthermore, the cavities and the high-density mass block inside the high-damping polymer are located at the center of the unit cell in the horizontal direction and are axially symmetrically distributed along the longitudinal axis of the unit cell in the incident direction of the sound wave.

[0028] Furthermore, the height of the high-damping polymer in the unit cell is 50 mm, which is made of modified rubber or modified polyurethane, with an elastic modulus of 20 to 30 MPa and an isotropic loss factor of 0.5 to 0.6.

[0029] Furthermore, the thickness of the upper and lower low water absorption fiber-reinforced resin panels is 1 to 2 mm, which is made of continuous quartz fiber or glass fiber-reinforced unsaturated vinyl resin material. Due to the large water absorption characteristics of conventional materials such as glass fiber-reinforced epoxy, the seawater infiltrating in the long-term use will accelerate the corrosion of polymer materials or metal materials, thus affecting the performance and service life of the sound-absorbing material. The unsaturated vinyl resin adopted in the present invention has obvious hydrophobicity. On the basis of having good mechanical properties and sound transmission properties, it can effectively reduce the water absorption rate of the resin panel and is easy to prepare resin panels with different thicknesses by vacuum forming process. The quartz fiber or glass fiber-reinforced unsaturated vinyl resin panel also has good rigidity, which can make the water pressure load act on the unit cell more evenly, thus avoiding the situation that the pure polymer unit cell is collapsed under high water pressure.

[0030] The above is the specific implementation of the present invention. In the prior art, the cavities inside the sound-absorbing polymer include cylindrical, conical or horn-shaped cavities and their combined forms. However, the cavity size is designed and optimized according to the steel backing conditions, and there are still great limitations in the low-frequency broadband sound-absorbing performance. Moreover, its surface has no panel protection and needs to be attached to the surface of the steel shell during use. The underwater sound-absorbing metamaterial resistant to the marine environment proposed by the present invention focuses on designing the sound-absorbing structure under the condition of water backing at both ends. A fiber-reinforced resin panel with low water absorption rate is used to protect the damping polymer from corrosion. In the sound-absorbing structure design, a combination form of small-diameter and large-length cavities in the upper layer and large-diameter and small-thickness cavities in the lower layer is adopted. At the same time, by reducing the distance between the end face of the mass block and the end faces of the upper and lower cavities, the resonance effect of the mass block is enhanced, thereby obtaining good low-frequency broadband sound-absorbing performance. This sound-absorbing structure also has excellent low-frequency broadband sound-absorbing performance in the steel backing. The underwater sound-absorbing metamaterial resistant to the marine environment proposed by the present invention has a simple structure, easily available raw materials, a mature preparation process, and strong designability of geometric parameters such as cavities, so the implementation feasibility is high. Two examples of specific implementation using the present invention are given below.

[0031] Example 1

[0032] The side length of the unit cell is 48 mm, the thickness of the damping polymer is 50 mm, the density is 960 g / cm 3 , the Young's modulus is 20 MPa, the Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.6. The thicknesses of the upper and lower fiber-reinforced resin panels are the same, with an elastic modulus of 20 GPa, a Poisson's ratio of 0.15, and a density of 1600 g / cm 3 . The characteristic density of water is 1000 kg / m 3 , and the sound speed is 1500 m / s. The distance from the bottom surface of the lower cavity to the panel is 3.6 mm, the radius is 14 mm, and the height is 2.4 mm; the distance from the bottom surface of the upper cavity to the lower panel is 12.5 mm, the radius is 2 mm, and the height is 26.25 mm; the mass block is made of steel, with a radius of 1.8 mm and a height of 5.33 mm, and the distances from its upper and lower end faces to the end faces of the upper and lower cavities are 0.585 mm respectively.

[0033] The acoustic performance of the periodic unit cell structure in the environment of water backing at both the acoustic wave incident end and the outgoing end is analyzed by the finite element method. Figure 3 and Figure 4It is the absorption, reflection, and transmittance of the underwater sound-absorbing metamaterial when the thicknesses of the fiber-reinforced resin panels in Example 1 are 1 mm and 2 mm respectively in the frequency range of 1 - 10 kHz. It can be seen that when the panel thickness is 1 mm, the sound wave absorption rate of the sound-absorbing metamaterial is greater than 0.74 in the frequency range of 3 - 10 kHz, and the average absorption coefficient in the frequency range of 1 - 3 kHz is 0.53; when the panel thickness is 2 mm, the sound wave absorption rate of the sound-absorbing metamaterial is greater than 0.75 in the frequency range of 3 - 10 kHz, and the average absorption coefficient in the frequency range of 1 - 3 kHz is 0.45, indicating good low-frequency broadband sound absorption effect. Figure 5 It is the comparison curve of the absorption rates of the sound-absorbing metamaterial and the damping polymer material. It can be seen that the pure damping polymer has good sound absorption performance at high frequencies above 6 kHz, but the sound absorption coefficient is small at low frequencies. By comparison, it can be known that the use of a 2-mm low water absorption fiber-reinforced resin panel in the present invention has little effect on the sound absorption performance of the damping polymer, indicating its good sound transmission performance, and at the same time is beneficial to increasing the hydrostatic pressure resistance of the underwater sound-absorbing metamaterial; through the sound absorption structure design, the absorption rate in the low frequency, especially in the frequency range of 2 kHz - 6 kHz, is significantly improved, indicating that the underwater sound-absorbing metamaterial proposed in the present invention has good low-frequency broadband sound absorption performance.

[0034] Example 2

[0035] The side length of the unit cell is 48 mm, the thickness of the damping polymer is 50 mm, and the density is 960 g / cm 3 , the Young's modulus is 30 MPa, the Poisson's ratio is 0.49, and the equivalent isotropic loss factor is 0.5. The thicknesses of the upper and lower fiber-reinforced resin panels are the same, with an elastic modulus of 20 GPa, a Poisson's ratio of 0.15, and a density of 1600 g / cm 3 . The characteristic density of water is 1000 kg / m 3 , and the sound velocity is 1500 m / s. The distance from the bottom surface of the lower cavity to the panel is 1 mm, the radius is 15 mm, and the height is 1.4 mm; the distance from the bottom surface of the upper cavity to the lower panel is 12.5 mm, the radius is 1.5 mm, and the height is 16 mm; the mass block is made of steel, with a radius of 0.75 mm and a height of 8 mm, and the distances from its upper and lower end surfaces to the upper and lower cavity end surfaces are 1 mm respectively.

[0036] The acoustic performance of the periodic unit cell structure in the water backing environment at both the sound wave incident end and the outgoing end is analyzed by the finite element method. Figure 6It is the absorption, reflection and transmittance of the underwater sound-absorbing metamaterial when the thickness of the fiber-reinforced resin panel in Example 2 is 2 mm in the range of 1-10 kHz. It can be seen that the sound wave absorption rate of the sound-absorbing metamaterial is greater than 0.85 in the frequency range of 3-10 kHz, and the average absorption coefficient in the frequency range of 1-3 kHz is 0.67, indicating that it has good low-frequency broadband sound absorption effect. Figure 7 It is the comparison curve of the absorption rates of the sound-absorbing metamaterial and the sandwich damping polymer material. It can be seen that the pure damping polymer has good sound absorption performance at high frequencies above 6 kHz, but the sound absorption coefficient is small at low frequencies. Through the sound-absorbing structure design, the absorption rate at low frequencies, especially in the frequency range of 2.5 kHz to 6.5 kHz, is significantly improved, indicating that the underwater sound-absorbing metamaterial proposed by the present invention has good low-frequency broadband sound absorption performance. Generally speaking, the underwater sound-absorbing metamaterial proposed by the present invention has good integrated performance of light weight and low-frequency broadband sound absorption.

Claims

1. An underwater sound-absorbing metamaterial, characterized in that, It includes periodically arranged unit cells. The unit cell is a sandwich structure. Along the incident and propagation direction of the sound wave, the upper and lower layers are low water absorption fiber-reinforced resin panels, and the middle is a high-damping polymer embedded with a cavity and a high-density mass block. The high-damping polymer internally contains two cylindrical cavities, the upper and lower ones. The high-density mass block is located between the two cavities and is not directly connected to the cavities. The diameter of the upper cavity is smaller than that of the lower cavity, and both are larger than the diameter of the high-density mass block. The ratio of the height of the high-density mass block to the distance between the upper and lower cavities is 0.8 to 0.9; The combination form of the cavities with a small diameter and large length in the upper layer and a large diameter and small thickness in the lower layer can achieve the impedance gradient change inside the sound-absorbing polymer and increase the scattering and waveform conversion effects of the cavities on the sound wave. Different resonance absorption frequencies also help to improve the sound absorption frequency band; The cavities and the high-density mass block inside the high-damping polymer are located at the center of the unit cell in the horizontal direction and are axially symmetrically distributed along the longitudinal axis of the unit cell in the incident direction of the sound wave; The height of the high-damping polymer in the unit cell is 50 mm, which is made of modified rubber or modified polyurethane, with an elastic modulus of 20 to 30 MPa and an isotropic loss factor of 0.5 to 0.6; The thickness of the upper and lower low water absorption fiber-reinforced resin panels is 1 to 2 mm, which is made of continuous glass fiber-reinforced unsaturated vinyl resin material.

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